[OpenMP] Fix balanced affinity so thread's private affinity mask is updated

Balanced affinity only updated the thread's affinity with the operating system.
This change also has the thread's private mask reflect that change as well so
that any API that probes the thread's affinity mask will report the correct
mask value.

Differential Revision: https://reviews.llvm.org/D52379

llvm-svn: 343142